1. Field of the Invention
The present invention relates to a sheet feeding apparatus and an image forming apparatus, and more particularly to the sheet feeding apparatus that separates and feeds a sheet by blowing air to a sheet.
2. Description of the Related Art
Conventionally, an image forming apparatus such as a printer, or a copying machine, includes a sheet feeding apparatus that feeds a sheet one by one from a sheet storing portion storing plural sheets. As the sheet feeding apparatus as described above, there is an air feeding system in which air is blown to the end of a sheet bundle stacked onto a tray in a sheet storing portion so as to blow up plural sheets, and only one sheet is absorbed to an absorption conveyance belt arranged above the sheet storing portion.
The sheet feeding apparatus includes a trailing end restricting member that restricts the trailing end of the sheet bundle stacked onto the tray, and a trailing end pressing member that is provided to the trailing end restricting member to be capable of moving in the vertical direction. The trailing end pressing member presses the trailing end of the sheet, which is blown up by the blown air with the position of the trailing end being restricted by the trailing end restricting member, with a fixed force from above. The position of the top surface of the uppermost sheet is detected based on the position of the trailing end pressing member in the vertical direction, whereby the lifting and lowering of the tray is controlled in order that the distance between the top surface of the uppermost sheet and the absorption surface of the absorption conveyance belt falls within a fixed range.
Since the trailing end pressing member is provided, the trailing end of the sheet is pressed by the trailing end pressing member, even when the uppermost sheet is blown up by the air blown from a leading end separation duct located at the side of the leading end of the sheet. As a result, only the central part of the uppermost sheet in the width direction is separated from the second sheet, and when the separated uppermost sheet is absorbed by the negative pressure of the absorption conveyance belt, a gap is formed between the uppermost sheet and the second sheet with the closed trailing end.
Since the gap described above is formed, air flowing through the gap flows all over the portion between the uppermost sheet and the second sheet. Consequently, the uppermost sheet and the second sheet can effectively be separated from the leading end of the sheet to the trailing end thereof, whereby the separation property of the sheet is enhanced. This technique is described in U.S. Pat. No. 7,540,489.
In the conventional sheet feeding apparatus, the trailing end of the sheet is restricted by the trailing end restricting plate, and pressed by the trailing end pressing member from above, so that, when loosening air is blown from the leading end of the sheet, the sheet is pushed toward the downstream side in the blowing direction. When a sheet having a small rigidity (degree of rigidity), e.g., a sheet called a thin sheet having a basis weight of 50 g/m2 or less, is fed, the sheet blown up by the loosening air blown from the leading end is pushed backward by the loosening air to be shifted backward. This is caused because of the reason described below. Specifically, since the rigidity of the sheet is small, the central part rises when the leading end of the sheet is pushed by the air with the trailing end being restricted. This condition is illustrated in FIG. 22. A sheet S stacked on a tray 12 is restricted by a trailing end pressing member 17 provided to a trailing end restricting portion 13. When loosening air and separation air are blown to the leading end of the sheet stacked onto the tray 12 from an air blowing portion 152 from a direction C or direction D, the central part of the uppermost sheet, which is blown up, might rise, in case where the rigidity of the sheet is small.
When the shift amount of the sheet Sb, which is the following sheet of the blowing-up uppermost sheet Sa, to the back of the uppermost sheet Sa is small, the leading end of the following sheet Sb might be exposed to the absorption conveyance belt 21. When the absorption conveyance belt 21 performs an absorption conveyance with this state, the absorption conveyance belt might absorb and convey the following sheet Sb together with the uppermost sheet Sa, which might cause a double feed. When the double feed described above is caused, a defective feeding such as skew feeding, or corner bending, might be generated. A defective image is generated, when a sheet is fed to an image forming portion with the double-feed state.